Water based inks have so far had the problem that if moisture thereof is evaporated, for example, dissolved substances and dispersed components contained in the inks are concentrated, deposited, dried and solidified to bring about clogging at the tips of the writing instruments and to increase a viscosity of the inks, thereby causing troubles in writing.
In this connection, it is proposed to add to a water based ink for a ball point pen, scarcely volatile dye-solubilizing agents or dye-dissolving aids such as urea, thiourea, polyhydric alcohols or derivatives thereof, tetrahydrofurfuryl alcohol, 4-methoxy-4-methylpentane-2-one, ethylene oxide adducts of p-toluenesulfonamide and thiodiethanolalkynolamine.
However, these urea, thiourea and polyhydric alcohols have not been able to provide satisfactory water based inks because of the reasons that they have an insufficient drying prevention effect or they have toxicity and are unsuitable for additives and that they allow a viscosity of the ink to grow large to bring about inferior follow-up of the ink.
Proposed in Japanese Patent Application NO. 9-181391 by the present applicant was a pseudoplastic water based ink for a ball point pen comprising a colorant, water, a water-soluble organic solvent and a viscosity-controlling agent and reducing dextrin or reducing maltodextrin. These inks using a water-soluble organic solvent such as glycols, glycerin and glycol ethers are increased in feathering of the drawn lines. Further, they do not provide sufficient lubricity, so that a ball point pen using them is increased in frictional resistance between the ball and the ball housing to abrade the ball housing and therefore has the defect that the writing property is notably reduced, such that the ball sinks and the ink is discharged unevenly.
Further, proposed in Japanese Patent Application Laid-Open No. 8-12916 is an ink comprising titanium oxide, a thickening water-soluble resin, a binder, a sugar alcohol and water, wherein a viscosity value is 6,000 to 50,000 mPa.multidot.s, and a viscosity index obtained according to a difference in a revolution of a viscometer is 2 or more. However, even an ink having such a high viscosity index does not solve the problems of a follow-up property in quick writing and splitting of the drawn lines. Further, it contains only a white pigment as a colorant and has the problem that a marking ability is inferior.
Further, it is proposed in Japanese Patent Application Laid-Open No. 5-186730 to add an alkylene oxide adduct of polyglycerin in order to improve feathering. However, though the adding of the alkylene oxide adduct of polyglycerin improves the feathering resistance and the writing feeling, the drying prevention effect at a pen tip is not provided. Accordingly, there remains unsolved the problem that when the pen is left standing with the cap off, moisture is vaporized from the pen tip and the pen tip is dried, so that the writing property is reduced.
An object of the present invention is to solve the problems described above, that is, to provide a water based ink for a ball point pen which is good in feathering resistance and writing feeling and has high discharge stability of the ink and which is excellent in a drying prevention effect at a pen tip.